The field of display device is very important for the information and communication industry. Recently, more advanced performance in this field is asked for in accordance with the development of information and communication technology. Display can be divided into luminescent type and non-luminescent type. The luminescent type of display comprises Cathode Ray Tube (CRT), Electroluminescence Display (ELD), Light Emitting Diode (LED), Plasma Display Panel (PDP), etc. The non-luminescent type of display comprises Liquid Crystal Display (LCD), etc.
The luminescent and non-luminescent type of displays have such basic performances as operation voltage, consumption power, brightness, contrast, response rate, life time, etc. However, LCD that has been widely used up to now has some problems in the above basic performances in regard to response rate, contrast, and sight dependency. Thus, the LED-using display is anticipated to take the place of next-generation display device by solving the above LCD problems and by providing such many advantages as fast response speed, no need for back light due to self-emission, and excellent brightness.
However, LED is mainly used with a crystal form of inorganic material, and so is hard to be applied to a large size of electroluminescent device. In addition, the electroluminescent device using inorganic material needs more than 200 V of operation voltage and is very expensive. However, Eastman Kodak reported in 1987 that the company manufactured a device made with a material having π-conjugate structure such as alumina quinine, and thereafter, the electroluminescent device study using organic material has been more active.
The electroluminescence device (EL device, below) can be divided into inorganic EL device and organic EL device depending on what material is used to form the emission layer (emitter layer).
The organic EL device is a self-emitting type of device that electrically excites fluorescent organic compound, is superior to the inorganic EL device in brightness, operation voltage, and response rate, and also can emit multi-color.
In addition, the organic EL device is a luminescent device to emit in low voltage current, and has superior properties such as enhanced brightness, high speed of response, wide viewing angle, plane luminescence, slim type, and multi-color luminescence.
Thus, the organic EL device is expected to be applicable to a full-color plat panel display due to such superior properties that cannot be found in other displays.
C. W. Tang et al. reported the first practical device performance of the organic EL device in Applied Physics Letters, vol. 51 (12) pp 913-915 (1987). They developed a thin film (a hole transport layer) of laminated structure formed by diamine analogues as organic layer and a thin film (an electron transport layer) formed by tris(8-quinolinolate)aluminum (Alq3, below). The laminated structure can lower the injection barrier of electron and hole from both electrodes to the organic layer, and also can enhance the re-combination probability of electron and hole from the inner organic layer.
Later, C. Adachi et al. developed an organic EL device having an organic luminescent layer with three-laminated structure of hole transport layer, emission layer, and electron transport layer [Japanese Journal of Applied Physics, vol. 27 (2), pp L269-L271 (1988)], and two-laminated structure of hole transportable emission layer and electron transport layer [Applied Physics Letter, vol. 55 (15), pp 1489-1491 (1989)], and showed that the optimization of device property can be achieved by constructing a multi-layer structure suitable for materials and combination thereof.
The organic EL comprises a first electrode (anode), a second electrode (cathode), and organic luminescent media. This organic luminescent media have at least two separate organic luminescent layers, i.e. one layer to inject and transport electron, and the other layer to inject and transport hole into the device. In addition, another multi-layer of thin organic film can be involved. The above layers to inject and transport electron and hole each can be divided into an electron injection layer, an electron transport layer, a hole injection layer, and a hole transport layer. In addition, the organic luminescent media can further include an emission layer besides the above layers.
The simple structure of organic EL device comprises a first electrode/an electron transport layer, and an emission layer/a second electrode. In addition, the structure of organic EL device can be separated into a first electrode/a hole injection layer/a hole transport layer/an emission layer/an electron transport layer/an electron injection layer/a second electrode.
The operation principle of the organic EL device having the above structure is as follows.
If the voltage is applied to the anode and cathode, the hole injected from the anode is transferred to the emission layer via the hole transport layer. Meanwhile, the electron is injected from the cathode to the emission layer via the electron transport layer. The hole and electron are re-combined in the emission layer to form exiton. The exiton is changed from the excitation state to the basic state, and thereby the fluorescent molecule of the emission layer becomes luminescent to form images.
At present, the material conventionally used for the hole transport layer is triphenylamine analogues. In addition, organic metal complex compounds or heterocyclic compounds are used for the electron transport layer. Organic compounds or organic metal complex compounds are solely used for the emission layer or as host of the emission layer. When organic compounds or organic metal complex compounds are used as host of the emission layer, organic luminescent materials or organic luminescent materials of metal complex type are used as dopant to control the color of luminescence.
The maximum quantum efficiency of luminescent materials used in an organic EL device is about 5% by theoretical calculation. If such low quantum efficiency can be enhanced, the life time of the device can be increased. When the molecule is fallen from the monoplet excitation state to the basic state, it is called generally as fluorescence. On the other hand, when the molecule is fallen from the triplet excitation state to the basic state, it is called as phosphorescence. In case of fluorescence, the maximum efficiency emitted from the basic state of molecule is about 25%. In case of phosphorescence, that is about 75%. Therefore, the phosphorescence materials having high luminescence efficiency have been applied to an organic thin layer, especially the emission layer of organic EL device, but the suitable materials for the organic thin layer have not been developed.
One practical method for the full-color display is to develop a material having high luminescence efficiency for the organic thin layer, especially the emission layer of organic EL device. Thus, study has been made for iridium metal complex organic compounds as phosphorescence materials for the organic EL device. The organic EL device using Such material as dopant of the emission layer has known to show high luminescence efficiency at operation [see Nature, vol. 403, pp 750˜753 (2000)].
Iridium metal complex organic compounds to constitute the emission layer have a different luminescent color in accordance with the molecular structure of ligand. In this case, the emission layer only comprises iridium metal complex organic compounds of phosphorescence materials or of phosphorescence materials as dopant. However, phosphorescence materials having practical luminescence efficiency have not been developed.
In view of the above, the present inventors have conducted intensive studies to develop novel phenyl pyridine-iridium metal complex compounds of formula (1) which have practical luminescence efficiency, and completed the present invention.